The newer engines place increased demands on the lubricants to be employed. In the past a number of different additives has been added to lubricating oils to improve properties such as viscosity index and dispersancy. One such additive added to lubricating oils to improve viscosity index is a two-block copolymer having the general configuration A-B where A is styrene and B is hydrogenated isoprene. See generally U.S. Pat. Nos. 3,763,044 and 3,772,196. A VI improver having greatly improved mechanical shear stability is the selectively hydrogenated star-shaped polymer disclosed in U.K. Patent Specification 1,575,507. Employing a single additive that improves a number of lubricant properties can achieve significant reductions in cost, and also circumvent the compatibility problems that can arise between the various additives and/or base oil. However, in attempting to improve more than a single lubricant property, care must be taken in not causing the deterioration of other properties. For example, the use of an oxidation step to attach polar groups to the polymer backbone in U.S. Pat. No. 3,864,268 leads to reduced lubricant stability by introducing sites for oxidative attack.
U.S. Pat. No. 4,141,847 describes hydrogenated conjugated diene star polymers and block copolymers having grafted dicarboxylic acid anhydride groups which are reacted with an amine to make ashless oil soluble additives having both dispersant and viscosity index improving properties.
U.S. Pat. No. 4,517,104 describes ethylene/.alpha.-olefin copolymers having grafted dicarboxylic acid anhydride groups which are reacted with a polyamine and an alkenyl succinic anhydride to make oil soluble viscosity index improvers having both dispersant and varnish inhibition properties.